Component of glass-like carbon for CVD apparatus and process for production thereof

ABSTRACT

Disclosed herein is an inner tube of glass-like carbon for CVD apparatus and a process for production thereof. The inner tube has its surface roughened without increase in metal impurities which cause particles. It has improved adhesion to CVD deposit film and also has a high degree of roundness. The surface roughness (on both the inner and outer surfaces) is 0.1-10 μm measured according to JIS B0601. The concentration of metal impurities (iron, copper, chromium, sodium, potassium, calcium, magnesium, and aluminum) in the surface is less than 50×10 10  atoms/cm 2 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a component of glass-like carbonfor CVD (Chemical Vapor Deposition) apparatus. Although thisspecification deals mainly with the inner tube for CVD apparatus, thepresent invention also covers various components to be arranged insideof CVD apparatus.

[0003] 2. Description of the Prior Art

[0004] Production of semiconductor devices generally relies on so-calledCVD process which involves gas-phase chemical reactions of reactantgases to deposit silicon, silicon nitride, etc. in the form of thin filmon silicon wafers. In this process, silicon wafers are placed in aninner tube (shown in FIG. 1) for their uniform heating and controlledflow of reactant gases.

[0005] An inner tube for CVD apparatus is required to have gooddurability (heat resistance at 500° C. or above and corrosion resistanceto reactant gases) under ordinary CVD conditions. It is also required togive off as little dust and impurity gas as possible. These requirementshave conventionally been met by an inner tube made of quartz.

[0006] CVD process is based on the principle that heated reactant gases(as raw materials) decompose or react with one another to form film(such as polysilicon film and silicon nitride film) on silicon wafers.During CVD process, decomposition or reaction products of reactant gasesdeposit on the surface of the inner tube. The inner tube carrying suchdeposits (polysilicon, silicon nitride, etc.) is used repeatedly withoutreplacement to maintain productivity. After continued use, the innertube becomes covered with thick deposits, which eventually peel off fromthe inner tube to give fine particles. Such fine particles stick tosilicon wafers, thereby reducing yields. Incidentally, “particles”denote particulate defects which are detected when wafers are examinedby an optical tester.

[0007] One way to eliminate particles originating from deposit thatpeels off from the inner tube is to periodically demount and clean theinner tube to remove CVD deposit. Cleaning employs chemical solutionslike hydrofluoric acid and nitric acid.

[0008] Such cleaning operation, however, reduces productivity andincreases production cost. Therefore, the inner tube should permit CVDdeposit to firmly adhere to it so that accumulated CVD deposit will notgive rise to particles. Adhesion of CVD deposit to the inner tube isaffected by the dimensional change of the inner tube which occurs whenthe CVD chamber is cooled and heated to unload and load wafers,respectively. The inner tube should ensure good adhesion despite itsdimensional change, due to such cooling/heating cycles. Moreover, sincecleaning procedure to remove CVD deposit is unavoidable, it is essentialfor the inner tube to have good corrosion resistance to cleaningsolutions as mentioned above. Unfortunately, conventional inner tubesmade of quartz are insufficient in adhesion to CVD deposit and corrosionresistance to cleaning solutions.

[0009] Under these circumstances, the present inventors previouslydeveloped a new inner tube of glass-like carbon for CVD apparatus whichprevents the occurrence of particles, resists corrosion by cleaningsolutions, and meets other requirements mentioned above, and they filedan application for patent (Japanese Patent Laid-open No. 3504/2001).

[0010] There is another known way to improve adhesion to CVD deposit forcomponents of glass-like carbon or components of graphite coated withglass-like carbon. It achieves its object by blast finishing whichroughens the surface of the component. (Japanese Patent Laid-open No.342068/2001 and Japanese Patent Publication No. 86662/1994) It is alsoknown that the inner tube and other components for CVD apparatus maycontain impurities to contaminate wafers during CVD process and thusinduced contamination causes resulting devices to malfunction.Therefore, the concentration of metal impurities in the surface ofcomponents should be no more than 10×10¹⁰ atoms/cm², preferably no morethan 5×10¹⁰ atoms/cm². In fact, a value lower than 5×10¹⁰ atoms/cm² hasbeen achieved in the case of a commercial dummy wafer made of glass-likecarbon prepared under adequately controlled conditions. This dummy waferhas a mirror-finished surface with a surface roughness no higher than0.1 μm. As with this dummy wafer, the inner tube of glass-like carbonfor CVD apparatus is required to have a surface metal concentration nohigher than 5×10¹⁰ atoms/cm².

[0011] Incidentally, the surface metal concentration is determined inthe same way as used for determination of metal on the surface ofsilicon wafers. The procedure starts with extracting metal from thesample with a mixed solution of 2% hydrofluoric acid and 2% hydrogenperoxide. Then the extract is analyzed by ICP-MS (inductively-coupledplasma mass spectrometry). The amount of metal thus determined isexpressed in terms of the number of atoms per unit area of the sampleused for extraction (atoms/cm²).

[0012] By the way, a tube of glass-like carbon is obtained by preparinga resin tube and then heating it in an inert atmosphere. The heatingtemperature is usually higher than 800° C., preferably 1000-1200° C.,more preferably 1300-2500° C. During heating for carbonization, theresin tube greatly shrinks (about 20% by volume). This shrinkage makesit difficult to obtain a completely round tube of glass-like carbon.

[0013] There was proposed a method of achieving roundness by inserting acylindrical graphite core in a resin tube before carbonization or in aglass-like carbon tube before heat treatment at high temperatures, asdisclosed in Japanese Patent Laid-open No. 189470/1999 and JapanesePatent Publication No. 189471/199. The core has an outside diameterequal to the inside diameter which the glass-like carbon tube would haveas the result of the resin tube shrinking after carbonization.

[0014] However, it turned out the above-mentioned conventional methodthat utilizes a cylindrical core does not always give a round glass-likecarbon tube.

OBJECT AND SUMMARY OF THE INVENTION

[0015] The technology disclosed in Japanese Patent Laid-open No.332504/2001 (mentioned above) permits one to effectively prevent theoccurrence of particles and to reduce the frequency of cleaningprocedures. This technology employs blast finishing to roughen thesurface of glass-like carbon inner tube for CVD apparatus to improveadhesion of CVD deposits, however it does not reduce particles to therequired level. This is due to the fact that surface rougheningincreases the content of metal impurities. At present, there is a demandfor an inner tube for CVD apparatus which gives off less impurities thanthat achieved by the above-mentioned technology.

[0016] Impurities such as particles occur when CVD deposit peels offfrom the surface of the inner tube as mentioned above. The presentinventors' investigation revealed that the film of CVD deposit becomesso thick as to cause cracking prior to peeling. Cracks in the surface ofCVD deposit film are shown in FIG. 5 (photograph taken by a scanningelectron microscope).

[0017] Incidentally, CVD deposit film forms not only on the surface ofan inner tube but also on the surface of various components exposed toCVD environment in CVD apparatus. Therefore, it is also necessary tosuppress impurities (particles) resulting from CVD deposit film whichforms on the surface of components other than inner tube.

[0018] It is known that carbonaceous materials such as glass-like carboncan be purified by heating at a high temperature (usually 2000° C. orabove) in a halogen-containing atmosphere, so that halogen penetratesinto carbon and vaporizes metal impurities from carbon. This processseems useful in decreasing metal impurities which may have contaminatedthe surface of an inner tube at the time of surface roughening.

[0019] On the other hand, the purifying process may cause dimensionaldistortion of the carbonaceous material. However, the inner tube for CVDapparatus should maintain sufficient roundness even after the purifyingprocess. A possible reason why the dimensional accuracy of glass-likecarbon tube is not so high is its anomalous volume change behavior.Namely, it shrinks up to about 1200° C., and then expands slightly athigher temperatures.

[0020] Incidentally, as with CVD deposit film mentioned above, metalimpurities exist not only on the surface of the inner tube but also onthe surface of various components in the CVD environment of CVDapparatus. Thus, it is necessary to reduce the amount of metalimpurities existing on the surface of components other than inner tube,thereby suppressing the formation of particles due to metal impurities.

[0021] Accordingly, it is an object of the present invention to providea component of glass-like carbon for CVD apparatus, which offers thefollowing advantages, and a process for production thereof. Improvedadhesion to CVD deposit. Ability to grow CVD deposit film sufficientlythick before cracking, thereby suppressing the occurrence of impurities(particles) that results from CVD deposit film peeling off. Adequatelyroughened surface which is made without increasing metal impurities thatgive rise to particles. Ability to prevent itself from giving off dust.It is another object of the present invention to provide a process forproducing a glass-like carbon tube having a high degree of roundness.

[0022] The gist of the present invention resides in a component ofglass-like carbon for CVD apparatus which is characterized by having avalue of surface roughness (Ra) ranging from 0.1 to 10 μm (measuredaccording to JIS B0601) and containing in its surface iron, copper,chromium, sodium, potassium, calcium, magnesium, and aluminum each in anamount less than 5×10¹⁰ atoms/cm². The surface roughness (Ra) specifiedin the present invention is a value measured according to JIS S0601,unless otherwise mentioned.

[0023] According to the present invention, the component of glass-likecarbon for CVD apparatus has a surface finished such that there exist atleast five pits, 1-10 μm in diameter, in the visual field, 50×50 μm,observed under a scanning electron microscope with a magnification of×1000, or a surface finished such that there exist elongated tinydepressions, 0.5-5 μm wide, whose total length is at least 50 μm, in thevisual field, 50×50 μm, observed under a scanning electron microscopewith a magnification of ×1000. Incidentally, the term “total length”means a value of the sum of the lengths of elongated tiny depressions(0.5-5 μm wide) present in the visual field.

[0024] According to the present invention, the component of glass-likecarbon for CVD apparatus is any of inner tube, wafer boat, susceptor,and nozzle used for CVD apparatus.

[0025] According to the present invention, the process for producing thecomponent of glass-like carbon for CVD apparatus is characterized by asurface roughening step and an ensuing purifying step. The surfaceroughening step may be a mechanical one, such as sand blasting andgrinding. The purifying step may be high-temperature heat treatment in ahalogen-containing atmosphere.

[0026] In the case where the component of glass-like carbon for CVDapparatus is a glass-like carbon tube, it is desirable that themechanical surface roughening be performed on both the inner and outersurfaces and then the purifying step be carried out.

[0027] According to the present invention, the process for producing acomponent of glass-like carbon for CVD apparatus is characterized bymechanical surface roughening and chemical surface etching, which may beperformed sequentially (in the order mentioned) or simultaneously.

[0028] The mechanical surface roughening may be accomplished by sandblasting or grinding, and the chemical surface etching may beaccomplished by thermal oxidation or electrolytic oxidation.

[0029] According to the present invention, a round tube of glass-likecarbon for CVD apparatus is obtained by steps of molding a raw materialresin into a tube, heating the resulting resin tube at 800-1300° C.,thereby converting it into a glass-like carbon tube, and heating theglass-like carbon tube at a temperature higher than 1500° C., togetherwith a roundness correcting jig attached to the outside of theglass-like carbon tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a schematic diagram illustrating a vertical low-pressureCVD apparatus, which is one of the CVD apparatus for producingsemiconductor devices by CVD process.

[0031]FIG. 2 is a schematic diagram illustrating the centrifugal moldingwhich is employed in the present invention.

[0032]FIG. 3 is a photograph taken by a scanning electron microscopewhich shows tiny pits existing in the surface of the inner tube for CVDapparatus which is covered by the present invention.

[0033]FIG. 4 is a photograph taken by a scanning electron microscopewhich shows elongated tiny depressions existing in the surface of theinner tube for CVD apparatus which is covered by the present invention.

[0034]FIG. 5 is a photograph taken by a scanning electron microscopewhich shows cracks in the surface of nitride film formed on the surfaceof an inner tube for CVD apparatus.

[0035]FIG. 6 is a graph showing the relation between the change inlength and the temperature of a piece of glass-like carbon which havebeen carbonized at 900° C.

[0036]FIG. 7 is a sectional view showing how a core (with carbon fiberfelt) is placed in a tube of phenolic resin.

[0037] FIGS. 8(a) and 8(b) are respectively a plan view and a sectionalview taken along A-A, which illustrate a roundness correcting jig usedin the present invention.

[0038]FIG. 9 is a sectional view showing a glass-like carbon tube(before high-temperature heat treatment) with a roundness correcting jigattached to its outside.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] According to the present invention, the inner tube of glass-likecarbon for CVD apparatus is characterized by having its inner and outersurface roughened, so that it has improved adhesion to CVD deposit owingto the anchoring effect produced by surface irregularities. According tothe present invention, the inner tube of glass-like carbon for CVDapparatus should have a value of surface roughness (R_(a)) ranging from0.1 to 10 μm (measured according to JIS B0601).

[0040] With a value of surface roughness smaller than 0.1 μm, the innertube of glass-like carbon does not have sufficient adhesion to CVDdeposit, with the result that CVD deposit suffers microcracking beforeit becomes sufficiently thick. This microcracking gives off a largenumber of particles, thereby making it necessary to replace the innertube frequently. With a value of surface roughness larger than 10 μm,the inner tube of glass-like carbon tends to lose its surface layer.

[0041] According to the present invention, the inner tube of glass-likecarbon for CVD apparatus has its inner and outer surfaces roughened(with R_(a) ranging from 0.1 to 10 μm) by surface roughening treatment.The surface roughening treatment should preferably be mechanical one.Particularly, sand blasting with fine powder is preferable to grinding,because it can achieve surface roughening without causing surfacedefects such as microcracking.

[0042] According to the present invention, the inner tube of glass-likecarbon for CVD apparatus is made of glass-like carbon, so that it has acoefficient of thermal expansion in the neighborhood of 3×10⁻⁶. Thisvalue is close to that of silicon nitride (3.4×10⁻⁶). This means thatthere is no significant dimensional change (and hence stress) due toheating between the inner tube and the silicon nitride deposit in theprocess of forming silicon nitride film. Such small stress contributesto improved adhesion between the inner tube and the silicon nitridedeposit.

[0043] According to the present invention, the inner tube of glass-likecarbon for CVD apparatus is characterizing by containing in its surfaceiron, copper, chromium, sodium, potassium, calcium, magnesium, andaluminum each in an amount less than 5×10¹⁰ atoms/cm². These metals areimpurities existing in the surface of the inner tube. If theconcentration of any of these metals exceeds 5×10¹⁰ atoms/cm², the innertube tends to give off a large number of particles resulting from thesemetals.

[0044] The present inventors continued their research on the cause ofimpurities (such as particles). As the result, it was found that theinner tube of glass-like carbon for CVD apparatus is not satisfactoryeven though it has its inner and outer surfaces roughened to an averageroughness (R_(a)) ranging from 0.1 to 10 μm (measured according to JISB0601). To be more specific, it was found that the surface roughened bysand blasting does not sufficiently improve adhesion to CVD depositfilm. It was also found that sand blasting leaves on the surfaceceramics fine powder or metal fine powder (used as abrasive media) orcarbon fine powder (given off by sand blasting) or stress (generatedduring sand blasting). They are the cause of particles falling onwafers.

[0045] The foregoing suggests that, in order to completely eliminateimpurities such as particles, it is necessary to contrive a method ofimproving adhesion to CVD deposit film more effectively than sandblasting and a method of preventing the occurrence of dust from theinner tube itself.

[0046] The present inventors' further investigation revealed that it ispossible to greatly improve adhesion between the inner tube and CVDdeposit film if the above-mentioned anchoring effect is enhanced bymechanical surface roughening (such as sand blasting that producesminute surface irregularities) and ensuring chemical surface etchingwhich forms extremely small surface irregularities with very littlechange in surface roughness (Ra) In what follows, “adhesion”occasionally implies adhesion between the inner tube surface and CVDdeposit film.

[0047] The inner tube according to the present invention shouldpreferably have extremely small surface irregularities on its surface asmentioned above. The surface irregularities are specified by (1) or (2)below so that adhesion between the inner tube and CVD deposit film isimproved. Improved adhesion permits the CVD deposit film to becomesufficiently thick before it suffers cracking. This prevents theoccurrence of impurities such as particles.

[0048] (1) The inner tube should have on its surface at least five pits,1-10 μm in diameter, in the visual field, 50×50 μm, observed under anSEM with a magnification of ×1000.

[0049] (2) The inner tube should have on its surface elongated tinydepressions, 0.5-5 μm wide, whose total length is at least 50 μm, in thevisual field, 50×50 μm, observed under an SEM with a magnification of×1000.

[0050] “Pits” in (1) above denotes approximately round depressionsexisting on the surface of the inner tube, as shown in the SEMphotograph in FIG. 3. They are indicated by arrows in FIG. 3(b). Theirdepth does not matter. (“Pits” may occasionally be referred to as“surface holes”.)

[0051] “Elongated tiny depressions” in (2) above are what is formedafter “pits” have grown. They are shown in an SEM photograph in FIG. 4.In FIG. 4(c), they are marked with a thick line for easy recognition.

[0052] The number of pits (surface holes) and the total length ofelongated tiny depressions, which are defined in the present invention,are an average value of three measurements obtained from a visual fieldof 50×50 μm, with a magnification mentioned above.

[0053] The surface structure specified in (1) above is such that thereexist at least five pits, 1-10 μm in diameter, in the visual fielddefined above. Such pits greatly improve adhesion between the inner tubeand CVD deposit film.

[0054] Pits less than five in the visual field are not enough to producea satisfactory anchoring effect (for adhesion between the inner tube andCVD deposit film). The number of pits should preferably be more than 10.

[0055] The number of pits in the visual field is not specificallylimited; however, it should preferably be less than 100 for the reasonsgiven below. Forming more than 100 pits requires surface chemicaletching for a long time. Also, as the number of pits increases, theinner tube becomes thin without significant improvement in adhesion.Thus, increasing the number of pits unduly is not desirable from theindustrial point of view.

[0056] Pits on the surface should have a diameter of 1-10 μm asspecified in (1) above for the reasons given below. With a diametersmaller than 1 μm, pits do not produce the anchoring effect forimprovement in adhesion to CVD deposit film. On the other hand, pitswith a diameter larger than 10 μm produce an adverse effect on adhesion.Presumably, this is because each pit has a relatively smooth surface andhence a large pit does not produce the anchoring effect as desired.

[0057] Elongated tiny depressions (0.5-5 μm wide) on the surface shouldhave a total length of at least 50 μm as specified in (2) above. If thetotal length is larger than 50 μm, elongated tiny depressions greatlyimprove adhesion between the inner tube and CVD deposit film.

[0058] If the total length is smaller than 50 μm, elongated tinydepressions do not fully improve adhesion (due to anchoring effect)between the inner tube and CVD deposit film. The total length shouldpreferably be larger than 100 μm.

[0059] The total length of elongated tiny depressions does not have anupper limit which is specifically established for improvement inadhesion. However, elongated tiny depressions having a total lengthexceeding 500 μm do not produce any significant additional effect.Therefore, the total length of elongated tiny depressions should be lessthan 500 μm from the industrial point of view.

[0060] Elongated tiny depressions should have a width ranging from 0.5to 5 μm as specified in (2) above for the reasons given below. Elongatedtiny depressions with a width smaller than 0.5 μm are too narrow toproduce the anchoring effect for improved adhesion to CVD deposit film.On the other hand, elongated tiny depressions with a width larger than 5μm causes the inner tube surface to give off fine powder.

[0061] As mentioned above, the inner tuber of the present inventionshould have either or both of the surface structure specified in (1) and(2) above.

[0062] The inner tube according to the present invention is produced bythe process explained in the following. A tubular form of glass-likecarbon, which is used as the inner tube in the present invention, can beproduced by an ordinary process, which comprises of a step of molding araw material resin and a step of carbonizing the resulting moldedobject. The carbonizing step may be preceded by a preliminary heatingstep to avoid distortion. In the case where a thermosetting resin(mentioned later) is used as a raw material resin, the curing treatmentmay function as the preliminary heating step. In this way it is possibleto cure the resin constituting the molded object and then carbonize theresin without serious thermal deformation.

[0063] To produce a tube of glass-like carbon, the molding step iscarried out so that the raw material resin is made into a hollowcylindrical form. The molding method in this case is not specificallyrestricted; it includes centrifugal molding, injection molding, andextrusion molding. Of these molding methods, centrifugal molding isparticularly desirable for the reasons given below. Centrifugal moldingcauses a molten resin to flow and cure inside a mold; therefore, itreadily yields tubular products with high dimensional accuracy and itpermits complete degassing (because both ends of the mold are open atthe time of molding). Incidentally, the raw material resin includes anyknown thermosetting resins such as phenolic resin and furan resin.

[0064] Details about centrifugal molding for raw material resin aredisclosed in Japanese Patent Laid-open No. 332504/2001.

[0065] The molding step to give a molded object (or resin tube) asmentioned above is followed by the carbonizing step which converts theresin tube into a glass-like carbon tube. Carbonization is usuallyaccomplished by heating at 800-2500° C. in an inert gas atmosphere (ornon-oxidizing atmosphere). The carbonizing step should be carried out insuch a way that the glass-like carbon tube maintains its round crosssection. This object is achieved by using a cylindrical core which isdisclosed in Japanese Patent Laid-open No. 179463/2002 and JapanesePatent Application No. 347393/2001. The method of improving roundnesswill be mentioned later.

[0066] In the case where a thermosetting resin is used as a raw materialresin, it is desirable to provide a curing step as mentioned above. Thecuring conditions vary from resin to resin; a phenolic resin is usuallycured in the air at 180-350° C. for 10-100 hours.

[0067] The inner tube according to the present invention may be producedby a method which includes, in addition to the molding and carbonizingsteps, a mechanical surface roughening step and a purifying step.

[0068] The mechanical surface roughening step is intended to form minuteirregularities on the surface of the glass-like carbon tube by any knownmechanical means. This step may be provided at any stage between themolding step and the carbonizing step. Mechanical surface roughening maybe performed on the molded object of raw material resin before or aftercuring or on the glass-like carbon tube after carbonization.Incidentally, carbonization may be accomplished in two stages at about900° C. and about 1200° C. In this case, mechanical surface rougheningmay be accomplished after the first stage or the second stage.

[0069] The method for mechanical surface roughening includes sandblasting and grinding. Sand blasting is preferable.

[0070] Abrasive powder for sand blasting is not specifically restricted,and any known one can be used. It includes ceramics powder (such asalumina powder and silicon carbide powder), metal powder, and glassbeads. The grain size of abrasive powder and the conditions of blasting(pressure and nozzle-work distance) may be properly selected accordingto the surface roughness required. The abrasive powder is usually finepowder with a grain size of about #220-800.

[0071] Grinding may be accomplished by using sand paper (#150-1000). Thegrain size and other conditions (grinding pressure etc.) may be properlyselected according to the surface roughness desired.

[0072] Mechanical surface roughening should preferably be performed onthe inner and outer sides of the tube simultaneously so as to minimizethe dimensional change caused by the purifying step which is carried outlater. If mechanical surface roughening is performed on the inner andouter sides separately, a comparatively large stress occurs in thesubsequent purifying step, and this makes it difficult to obtain aninner tube with sufficient roundness for CVD apparatus. (The reason forthis is not known.)

[0073] According to another preferred embodiment, mechanical surfaceroughening is accompanied by or followed by chemical surface etching.Chemical surface etching is intended to partly remove the surface of theglass-like carbon tube, thereby forming minute surface irregularities.This treatment removes fine powder and damages (such as cracking)resulting from mechanical surface roughening. Therefore, it prevents theinner tube from releasing dust.

[0074] Chemical surface etching is usually carried out after mechanicalsurface roughening. If this order is reversed, minute irregularitiesformed by chemical surface etching are destroyed by mechanical surfaceroughening. To be more specific, chemical surface etching is performedon the glass-like carbon tube on which minute irregularities have beenformed by mechanical surface roughening in any stage. However, it ispossible to perform mechanical surface roughening (such as sandblasting) on the glass-like carbon tube (which has not yet undergonemechanical surface roughening) in an environment for chemical surfaceroughening (for example, in an environment for thermal oxidation). Theresult in this case is that mechanical surface roughening and chemicalsurface roughening are carried out simultaneously.

[0075] The treatment for chemical surface roughening includes thermaloxidation, electrolytic oxidation, and chemical etching.

[0076] Thermal oxidation is carried out in such a way that the surfaceof the glass-like carbon tube is oxidized to give the surface structureas defined in (1) or (2) above. It is usually accomplished by heattreatment in an oxidizing atmosphere (air or oxygen) at 600-800° C. for0.5-10 hours.

[0077] Electrolytic oxidation is accomplished by applying electriccurrent across the glass-like carbon tube (as an anode) and a cathode ofplatinum, stainless steel, or nickel, which are immersed in anelectrolyte. The electrolyte is an aqueous solution of sodium hydroxide,calcium hydroxide, or ammonia. The concentration of the electrolyte(NaOH solution) is 0.1-2 M.

[0078] Electrolytic oxidation may be carried out under any unrestrictedconditions so long as the resulting glass-like carbon tube has thesurface structure as defined in (1) or (2) above. The degree of etchingis controlled by adjusting the amount of current, which is usually 5-500C/cm².

[0079] Chemical etching is accomplished by immersing the glass-likecarbon tube in a chemical solution capable of dissolving its surface.The chemical solution is exemplified by potassium or sodium dichromateaqueous solution and chromic acid mixture.

[0080] Electrolytic oxidation is preferable because it permits thedegree of etching to be controlled easily.

[0081] Incidentally, according to the present invention, the glass-likecarbon tube which has undergone mechanical surface roughening alone orin combination with chemical surface etching should have a value ofsurface roughness ranging from 0.1 to 10 μm, preferably from 0.3 to 3μm.

[0082] The glass-like carbon tube which has undergone surface rougheningsubsequently undergoes purifying treatment by heating at a hightemperature in a halogen-containing gas environment. This purifyingtreatment reduces the content of the metal impurities which may haveentered the surface of the glass-like carbon tube at the time ofmechanical surface roughening. After this purifying treatment, followedby surface cleaning treatment, the concentration of each of iron,copper, chromium, sodium, potassium, calcium, magnesium, and aluminum inthe surface is reduced to less than 5×10¹⁰ atoms/cm².

[0083] The present inventors' investigation revealed that if the surfaceroughness of the inner tube exceeds 10 μm, the purifying treatment doesnot reduce the metal concentration below 5×10¹⁰ atoms/cm², even afterthe surface cleaning treatment. A probable reason for this is thatexcessive surface roughening increases the effective surface area andhence the metal concentration per unit area of the geometrical surfaceof the inner tube increases. Therefore, the inner and outer surfaces ofthe inner tube for CVD apparatus should be roughened to a certain limitwhich is 10 μm, from the standpoint of reducing the metal concentrationin the surface. Incidentally, after the purifying treatment, the innertube should be washed with hydrofluoric acid, hydrochloric acid, orhydrogen peroxide in the usual way.

[0084] According to the present invention, the following method isemployed to improve the roundness of the glass-like carbon tube. Theprocess according to the present invention includes the steps of moldinga raw material resin into a tube and carbonizing the resin tube byheating in an inert atmosphere at 800-1300° C. In the carbonizing step,the resin tube shrinks and vitrifies to give a glass-like carbon tubewhich subsequently undergoes high-temperature heat treatment. Thecarbonizing step should be carried out in such a way that a cylindricalcore is placed inside the resin tube so that the tube remains roundduring heat treatment. Then, a roundness correcting jig is place aroundthe outside of the glass-like carbon tube (which does not yet undergohigh-temperature heat treatment). The glass-like carbon tube and theroundness correcting jig are heated together at a high temperature (say,1500° C. or above) in an inert atmosphere. During heat treatment at sucha high temperature, the roundness correcting jig undergoes thermalexpansion and the glass-like carbon tube also undergoes thermalexpansion as well as expansion due to structural change of carbon. Asthe result, the glass-like carbon tube, which has undergonehigh-temperature heat treatment, keeps a high degree of roundness, withits outer surface conforming to the inner surface of the roundnesscorrecting jig.

[0085]FIG. 6 is a graph showing the relation between the temperature andthe change in length in the case where the glass-like carbon sample(which does not yet undergo high-temperature heat treatment) is heated.The rate of heating is 200° C./h. The glass-like carbon sample is arectangular shape measuring 2 mm thick, 2 mm wide, 20 mm long, which hasbeen carbonized by heating at 900° C.

[0086] As FIG. 6 shows, the molded sample of glass-like carbon expandsuntil it is heated to about 950° C., shrinks while it is heated from950° C. to 1200° C., and expands again while it is heated above 1200° C.The slope of the second expansion (at 1200° C. and above) is steeperthan that of the first expansion (at 0-900° C.). At 0-900° C., themolded sample of glass-like carbon has a coefficient of linear expansionof about 3×10⁻⁶ (K⁻¹), while at 1200-1500° C., it has a coefficient ofthermal expansion of about 10×10⁻⁶ (K⁻¹). The difference between the twocoefficients is attributable to the change in structure that occurs at1200-1500° C. In other words, the molded sample of glass-like carbon(which has been carbonized at 900° C. in this example) expands when itis heated from 1200° C. to 1500° C. The mechanism of expansion is notfully elucidated; probably it is caused by the change in structure ofcarbon.

[0087] The foregoing suggests that carbonization to give the glass-likecarbon tube (which subsequently undergoes high-temperature heattreatment) should be carried out at lower than 1200° C., so that theglass-like carbon tube is heated for a long time at 1200-1500° C. in thehigh-temperature heat treatment. The upper limit of the carbonizingtemperature should be 1300° C., with possible variation of raw materialresin taken into consideration. The lower limit of the carbonizingtemperature should be 800° C. at which the resin tube vitrifies. Thislower limit should preferably be as close to 1200° C. as possible atwhich the material shrinks almost completely. (A molded object ofglass-like carbon which has been heated at 900° C. shrinks at 950-1200°C.) The reason for this is that if the glass-like carbon tube greatlyshrinks while it is heated through a temperature range from 950 to 1200°C., the glass-like carbon tube excessively separates from the roundnesscorrecting jig surrounding it and this lessens the subsequent roundnesscorrecting effect due to expansion. Incidentally, the reason why thehigh-temperature heat treatment should be carried out at 1500° C. orabove is that it does not fully produce the roundness correcting effectdue to expansion (resulting from the structural change mentioned above)if its temperature is lower than specified. The upper limit of thetemperature for high-temperature heat treatment is usually 2500° C.although it varies depending on the quality and usage temperature of theglass-like carbon tube desired.

[0088] The roundness correcting jig used in the present invention shouldbe formed from a material having good heat resistance and a coefficientof thermal expansion close to that of the glass-like carbon tube whichundergoes high-temperature heat treatment. The material includesgraphite, glass-like carbon, and carbon fiber, with the first beingpreferable.

[0089] The foregoing description is directed mostly to an inner tube ofglass-like carbon for CVD apparatus. The scope of the present inventioncovers not only inner tubes but also other components for CVD apparatus,such as boats to hold wafers, susceptors, nozzles, etc. They areprepared from a raw material resin by molding, carbonization, surfaceroughening, and surface chemical etching as in the case of inner tube.

[0090] As explained above for inner tubes, these components ofglass-like carbon for CVD apparatus have extremely tiny surfaceirregularities due to mechanical surface roughening and extremely smallsurface irregularities due to chemical surface etching. This surfacestructure produces the anchoring effect, contributing to improvedadhesion to CVD deposit film. In addition, the chemical surface etchingprevents the components from releasing dust from themselves. Theseproperties are very useful in production of semiconductor devices.

EXAMPLES

[0091] The invention will be described in more detail with reference tothe following examples, which are not intended to restrict the scopethereof and changes and modifications may be made without departing fromthe scope thereof.

[0092]FIG. 2 is a schematic sectional view showing an example of themold for centrifugal molding which is used in one example of the presentinvention. The mold 10 for centrifugal molding consists of a mold proper11, a bottom plate 12, and a flange stopper 13. The mold proper 11 is acylindrical body of stainless steel, measuring 325 mm in inside diameterand 1600 mm long. It has a detachable bottom plate 12 on one endthereof, which is driven by a motor through a belt. It also has adetachable annular flange stopper 13 on the other end thereof, whichprevents the thermosetting resin from leaking therefrom. This flangestopper 13 has an opening which permits reaction gases to escape. Themold proper 11 is of separable type (consisting of two sections) foreasy demolding. Centrifugal molding is accomplished by charging the moldproper 11 with a thermosetting resin and then rotating the mold assembly10 while heating it at a thermosetting temperature. Heating isaccomplished by electric heaters surrounding the mold proper 11.

Example 1

[0093] [Preparation of Glass-Like Carbon Tube]

[0094] Raw material resin: A preferred one for glass-like carbon is athermosetting resin such as phenolic resin and furan resin. This exampleemployed a commercial phenolic resin “PL4804” from Gun-ei Kagaku KogyôCo., Ltd., which had previously been vacuum-dried (below 5 wt % watercontent) at 65° C. for 6 hours under a reduced pressure of 10 Torr.Preparation of phenolic resin tube: A tube of phenolic resin was formedas follows by using a centrifugal molding machine equipped with the mold10. The mold 10 was charged with the phenolic resin in a prescribedamount. The mold 10 was rotated at 600 rpm for 5 hours, with its surfacetemperature kept at 100° C. The mold was cooled to room temperature fordemolding. Thus there was obtained a phenolic resin tube measuring 3 mmthick, 320 mm in outside diameter, and 1500 mm long. This procedure wasrepeated to prepare several samples.

[0095] Curing: The thus obtained phenolic resin tube was cured byheating at 300° C. for 200 hours. This curing is intended to prevent thephenolic resin tube from deforming during carbonization; therefore, itmay be omitted if there is no possibility of deformation.

[0096] Carbonization: The phenolic resin tube was carbonized by heatingat 1600° C. in an inert gas (nitrogen) atmosphere. Thus there wasobtained a glass-like carbon tube measuring 2.5 mm thick, 268 mm inoutside diameter, and 1265 mm long. Sand blasting: The glass-like carbontube thus obtained was exposed to a jet stream of a slurry of siliconcarbide in water, which was directed to both inside and outside thereof,so that both sides are roughened simultaneously. In this process, eachsample underwent roughening with silicon carbide varying in grain size,with other conditions (pressure and nozzle-surface distance) unchanged.Electrolytic oxidation: The glass-like carbon tube with its surfaceroughened underwent electrolytic oxidation by immersion in an aqueoussolution of sodium hydroxide (0.1 mol/L), with current applied tobetween the sample and a platinum sheet counter electrode (cathode).This step was followed by washing and drying in the usual way. Purifyingprocess: The glass-like carbon tube which had undergone electrolyticoxidation was heated (for purifying) at 2200° C. in achlorine-containing atmosphere. The purified sample was cleaned with amixture of 2% hydrofluoric acid and 2% hydrogen peroxide and then rinsedrepeatedly with ultrapure water and dried in a clean room.

[0097] Experiment Nos. 1 to 4

[0098] Four samples (Nos. 1 to 4) of inner tube for CVD apparatus wereprepared by performing the above-mentioned steps in the following order.Preparation of phenolic resin tube→Curing→Carbonization→Sandblasting→Electrolytic oxidation→Purifying.

[0099] Experiments Nos. 5 and 6

[0100] Two samples (Nos. 5 and 6) of inner tube for CVD apparatus wereprepared by performing the above-mentioned steps in the following order.Preparation of phenolic resin tube→Curing→Carbonization Purifying→Sandblasting→Electrolytic oxidation. (Sand blasting for No.5 and No. 6 wasperformed under the same condition as for No. 1 and No. 2,respectively.) The procedure for Nos. 5 and 6 is the same as that forNos. 1 and 2 except that the purifying step precedes the sand blastingand electrolytic oxidation steps.

[0101] Experiments Nos. 7 and 8

[0102] The sample No. 7 was prepared by the same procedure as forsamples Nos. 1 to 4, except that the sand blasting step for surfaceroughening employed finer silicon carbide particles. The sample No. 8was prepared by the same procedure as for samples Nos. 1 to 4, exceptthat the sand blasting step for surface roughening employed coarsersilicon carbide particles.

[0103] Experiment No. 9

[0104] The sample No. 9 was prepared by the same procedure as forsamples Nos. 1 to 4, except that the sand blasting step was performed intwo stages, the first one for inside and the second one for outside.

[0105] The thus obtained samples Nos. 1 to 9 were tested for thefollowing items. The results are shown in Table 1.

[0106] (1) Surface Roughness:

[0107] After surface roughening, the inner and outer surfaces of theinner tube for CVD apparatus were tested for surface roughness (R_(a))specified by JIS B0601 by using a contact probe-type roughness tester(made by Rank Tailor Co., Ltd.) The surface roughness (R_(a)) isexpressed in terms of an average value of measurements made in tworegions (in each of the inner and outer surfaces) in which there are noscratches due to handling.

[0108] (2) Number of Particles:

[0109] The number of particles released from the inner tube for CVDapparatus was counted in the following manner. The inner tube was placedin a vertical CVD apparatus which had been charged with silicon wafers.The CVD apparatus was fed with a mixed gas of dichlorosilane (SiCl₂H₂)and ammonia (NH₃) at 770° C. under reduced pressure. The step of formingsilicon nitride film (150 nm thick) on each silicon wafer was repeated100 times. After the first run and the hundredth run, the number ofparticles larger than 0.2 μm was counted. This procedure wasaccomplished by using a Surfscan (Model WH-1700, made by Topcon).

[0110] (3) Concentration of Metal:

[0111] The concentration of metal in the sample was determined by ICPmass spectrometry in the following manner. A small amount of a 50:50mixture of 2% hydrofluoric acid and 2% hydrogen peroxide is dropped ontothe surface of the silicon wafer to be analyzed. The solution on thesurface is allowed to stand for about 10 minutes and then recovered. Theconcentration of metal was analyzed by ICP-MS, and the figures areexpressed in terms of the number of metal atoms per unit area of thesilicon wafer. The chemicals used for analysis was ultrapure reagentsmade by Kanto Kagaku Co., Ltd. The apparatus used for ICP massspectrometry was SPQ9000SE made by Seiko Instruments Co., Ltd. Theapparatus was run under the condition (a) for iron, chromium, sodium,potassium, calcium, magnesium, and aluminum, and the condition (b) forcopper.

[0112] (a) high-frequency output: 0.8 kW, sampling depth: 8 mm, carriergas: 0.9 L/min, with PFA microflow nebulizer, chamber gas: 0.33 L/min.

[0113] (b) high-frequency output: 1.3 kW, sampling depth: 8 mm, carriergas: 0.9 L/min, with PFA microflow nebulizer, chamber gas: non.

[0114] (4) Roundness:

[0115] The maximum and minimum values of the outside diameter weremeasured at the upper end of the inner tube for CVD apparatus. Roundnesswas expressed in terms of difference between the maximum outsidediameter and the minimum outside diameter. TABLE 1 Surface Number ofparticles Experiment roughness Roundness Concentration of metal in thesurface (x 10¹⁰ atoms/cm²) After After No. Ra (μm) (mm) Fe Cu Cr Na K CaMg Al 1^(st) run 100^(th) run 1 0.13 0.9 <0.2 <0.2 <0.2 <0.2 <0.2 0.90.3 1.1 38 73 2 1.3 0.8 <0.2 <0.2 <0.2 0.5 0.4 0.9 0.5 2.1 35 58 3 2.61.2 <0.2 <0.2 <0.2 1.6 1.9 1.8 1.1 3.1 39 48 4 7.9 0.7 <0.2 <0.2 <0.20.4 0.5 2.3 1.6 1.7 48 82 5 0.32 0.8 7.8 1.4 1.2 10.6 11.2 13.5 16.313.9 336 — 6 1.3 0.8 5.9 2.4 1.6 13.7 10.5 12.1 11.6 16.2 415 — 7 0.081.1 <0.2 <0.2 <0.2 <0.2 <0.2 0.3 1.2 3.8 46 — 8 10.9 1.4 6.3 2.1 <0.215.8 9.9 13.4 16.1 13.8 442 — 9 2.4 3.2 0.8 0.3 <0.2 0.3 0.6 1.9 2.2 0.9— —

[0116] As Table 1 shows, in the case of samples Nos. 1 to 4, which havethe surface roughness and the surface metal concentration within therange specified in the present invention, the number of particles wasless than 60 after the first run of film forming operation and evenafter the hundredth run of film forming operation in which thecumulative film thickness reached 15 μm. This result is good forsatisfactory device yields.

[0117] In contrast, samples Nos. 5 and 6, which underwent sand blastingand electrolytic oxidation after the purifying step, were found to haveresidual metal impurities which had entered the surface of theglass-like carbon tube during its surface roughening by sand blasting.Therefore, they failed to meet the requirement of surface metalconcentration specified in the present invention. They gave off a largenumber of particles (presumably originating from metal impurities)throughout the film forming process. They were not suitable forproduction of semiconductor devices.

[0118] Sample No. 7, which underwent sand blasting with fine siliconcarbide grits, had a value of surface roughness which is lower than thatspecified in the present invention. It gave off a less number ofparticles in the initial stage of film forming process; however, thenumber of particles increased as the film forming process was repeated.It did not permit the film forming process to be repeated 100 timesowing to more particles than specified. These excessive particlesoriginated from microcracking in the CVD deposit film which becamethicker after the repeated film forming process. Sample No. 8, whichunderwent sand blasting with coarse silicon carbide grits, had a valueof surface roughness which is higher than that specified in the presentinvention. Because of its large surface roughness (10.9 μm) exceedingthe upper limit (10 μm), it had a surface metal concentration in excessof 5×10¹⁰ atoms/cm² despite the purifying process. It was not used forproduction of semiconductor devices because it gave off a large numberof particles in the first run of film forming process.

[0119] Sample No 9, which was poor in roundness (3.2 mm in terms ofdifference between the maximum and minimum outside diameters at the topof the tube) due to a comparatively large strain experienced in thepurifying process, was not suitable for the use as an inner tube of CVDapparatus because it took a very long time to be machined into a properdimension.

Example 2

[0120] [Preparation of Glass-Like Carbon Tube]

[0121] Samples of glass-like carbon tubes were prepared as follows froma commercial phenolic resin “PL4804” from Gunei Kagaku Kogyô Co., Ltd.,which had previously been vacuum-dried (below 5 wt % water content) at100° C. for 1 hour under a reduced pressure of 10 Torr. The phenolicresin was made into a tube by using a centrifugal molding machineequipped with the mold 10. Charged with 7 kg of the phenolic resin, themold 10 was turned at 600 rpm for 10 hours, with its surface temperaturekept at 120° C. so that the phenolic resin was melted. The mold wascooled to room temperature for demolding. Thus there was obtained aphenolic resin tube measuring 3 mm thick, 323 mm in outside diameter,and 1590 mm long.

[0122] The phenolic resin tube was cured by heating in the air at 250°C. for 10 hours. The cured phenolic resin tube was carbonized by heatingat 1600° C. in an inert gas (nitrogen) atmosphere. Thus there wasobtained a glass-like carbon tube measuring 2.5 mm thick, 268 mm inoutside diameter, and 1265 mm long.

[0123] Experiment No. 1

[0124] The sample of glass-like carbon tube mentioned above wassand-blasted with #400 alumina powder for mechanical roughening of theinner and outer surfaces thereof. After sand blasting, the sample had avalue of surface roughness of 0.6 μm.

[0125] Subsequently, the sample underwent electrolytic oxidation (forchemical surface etching) under the conditions shown in Table 2 in a0.1M aqueous solution of sodium hydroxide, with current applied tobetween the sample and a platinum sheet counter electrode (cathode).This step was followed by washing and drying in the usual way. The thustreated sample was found to have a value of surface roughness of 0.6 μm.In other words, the chemical surface etching did not change surfaceroughness.

[0126] The sample which had undergone electrolytic oxidation waspurified by heating at 2200° C. in a chlorine-containing gas. Thepurified sample was washed with a mixture of 2% hydrofluoric acid and 2%hydrogen peroxide and then rinsed with ultrapure water repeatedly. Itwas finally dried in a clean room. Thus there was obtained the desiredinner tube for CVD apparatus. It was examined for the surface metalconcentration in the same way as in Example 1. The result was no morethan 5×10¹⁰ atoms/cm² in all the samples.

[0127] Experiments Nos. 2 to 7

[0128] The samples underwent sand blasting in the same way as inExperiment No. 1. After sand blasting, the samples were found to have avalue of surface roughness of 0.6 μm. The samples underwent electrolyticoxidation in the same way as in Experiment No. 1, except that theconditions were changed as shown in Table 2. Electrolytic oxidation wasfollowed by washing, drying, and purifying. Thus there were obtainedinner tubes for CVD apparatus. They were found to have a value ofsurface roughness of 0.6 μm. This indicates that the chemical surfaceetching (electrolytic oxidation) did not change surface roughness. Theywere also examined for the surface metal concentration in the same wayas in Example 1. The result was no more than 5×10¹⁰ atoms/cm² in all thesamples.

[0129] The thus obtained inner tubes for CVD apparatus (numbered 1 to 7)were tested for the following items. The results are shown in Table 2.

[0130] (1) Surface Roughness:

[0131] The samples were tested for surface roughness in the same way asin Example 1.

[0132] (2) Pits and Elongated Tiny Depressions:

[0133] The samples were examined for their surface by observation underan SEM (×1000). Three arbitrary visual fields (50×50 μm) werephotographed. The number of pits (1-10 μm in diameter) in each visualfield was counted and the total length of elongated tiny depressions(0.5-5 μm wide) in each visual field was measured. Average values pervisual field were calculated.

[0134] (3) Thickness at Which Surface Cracking Occurs in CVD DepositFilm (Nitride Film):

[0135] Small pieces of the inner tube were set in a vertical type lowpressure CVD apparatus. The CVD apparatus was fed with a mixture gas ofNH₃ and SiCl₂H₂ at 780° C. so that nitride film was formed on thesurface of the inner tube. The surface of the nitride film formed on thepieces was observed under an SEM (×5000) in arbitrary ten visual fieldsat certain time intervals. The film thickness at which cracking began tooccur was recorded.

[0136] (4) Number of Particles:

[0137] The number of particles released from the inner tube for CVDapparatus was counted in the following manner. The inner tube was placedin a vertical CVD apparatus which had been charged with silicon wafers.The CVD apparatus was heated to 800° C. under reduced pressure withoutbeing fed with reactant gases. The wafers were remoyed from the CVDapparatus, and the number of particles on them was counted.

[0138] Incidentally, FIG. 3 is an SEM photograph showing the surface ofthe inner tuber for CVD apparatus which was produced under the samecondition as in Sample No. 4. FIG. 3(a) consists of two parts. The partbelow the broken line represents the surface which was masked at thetime of chemical surface etching so that it substantially underwent onlymechanical surface roughening (sand blasting). The part above the brokenline represents the surface which underwent mechanical surfaceroughening and subsequent chemical surface etching. FIG. 3(b) is apartial enlargement representing a visual field (50×50 μm) in FIG. 3(a).Arrows in FIG. 3(b) denote surface pits.

[0139]FIG. 4 is an SEM photograph showing the surface of the inner tuberfor CVD apparatus which was produced under the same condition as inSample No. 3. FIG. 4(b) is a partial enlargement representing a visualfield (50×50 μm) in FIG. 4(a). Elongated depressions are apparentlynoticed. In FIG. 4(c) elongated depressions are marked with thick linesfor easy recognition. TABLE 2 Total length of Amount of Number ofelongated Thickness at which Number of Experiment current applied pitsin depression surface cracking particles No. (C/cm²) surface (μm) occurs(μm) (per wafer) 1 10 4 55 10 115 2 40 7 60 12 97 3 80 10 260 18 53 4100 17 —* 22 62 5 200 32 —* 38 49 6 0 0 0 0.4 233 7 5 3 20 1.3 196

[0140] Samples Nos. 1 to 5, which meet the requirements for the numberof surface pits and the total length of elongated tiny depressions,permitted nitride film to grow to a substantial thickness before surfacecracking occurs. This means that they can be used continuously withoutdust release until a thick nitride film is formed thereon. In addition,they gave off very few particles during operation.

[0141] By contrast, samples Nos. 6 and 7, which do not meet therequirements for the number of surface pits and the total length ofelongated tiny depressions, have the following shortcomings.

[0142] Sample No. 6, which did not undergo chemical surface etching(electrolytic oxidation), caused the nitride film to crack soon afterthe start of operation. Cracking of nitride film necessitatesreplacement and cleaning of the inner tuber in the early stage and alsogives off many particles during operation.

[0143] Sample No. 7, which underwent electrolytic oxidation with areduced amount of current, has a less number of pits and a less totallength of elongated depressions. Therefore, it caused the nitride filmto crack soon after the start of operation. Cracking of nitride filmnecessitates replacement and cleaning of the inner tuber in the earlystage and also gives off many particles during operation.

[0144] Experiment No. 8

[0145] The glass-like carbon tube mentioned above underwent grindingwith sand paper (#240) for mechanical surface roughening on its innerand outer surfaces. This treatment gave a surface roughness of 2.1 μm.

[0146] Subsequently, the glass-like carbon tube underwent thermaloxidation in the air for 1 hour at varied temperatures shown in Table 3.This process did not change the surface roughness (2.1 μm).

[0147] The glass-like carbon tube, which had been thermally oxidized,underwent purifying treatment, followed by cleaning and drying, in thesame way as in experiments Nos. 1 to 7. The resulting inner tube for CVDwas tested for surface metal concentration in the same way as inExample 1. The result was less than 5×10¹⁰ atoms/cm² in all the samples.

[0148] Experiments Nos. 9 to 12

[0149] Samples of glass-like carbon tubes with grinding treatment wereprepared in the same way as in Experiment No. 8. They were found to havea value of surface roughness of 2.1 μm. They underwent thermal oxidationand purifying process in the same way as in Experiment No. 8, exceptthat the temperature was changed as shown in Table 3. The resultinginner tubes for CVD apparatus were found to have a value of surfaceroughness of 2.1 μm. This suggests that chemical surface etching(thermal oxidation) did not change surface roughness. They were alsotested for surface metal concentration in the same way as in Example 1.The result was less than 5×10¹⁰ atoms/cm² in all the samples.

[0150] The thus obtained inner tubes for CVD apparatus were tested forthe items in Experiments Nos. 1 to 7. The results are shown in Table 3.TABLE 3 Total length of Temperature Number of elongated Thickness atwhich Number of Experiment of thermal pits in depressions surfacecracking particles No. oxidation (° C.) surface (μm) occurs (μm) (perwafer) 8 650 6 58 8 92 9 675 21 114 16 87 10 700 29 —* 26 89 11 — 0 00.6 325 12 600 5 42 1.5 126

[0151] Samples Nos. 8 to 10, which meet the requirements for the numberof surface pits and the total length of elongated tiny depressions,permitted nitride film to grow to a substantial thickness before surfacecracking occurs. This means that they can be used continuously withoutdust release until a thick nitride film is formed thereon. In addition,they gave off very few particles during operation.

[0152] By contrast, samples Nos.11 and 12, which do not meet therequirements for the number of surface pits and the total length ofelongated tiny depressions, have the following shortcomings.

[0153] Sample No. 11, which did not undergo chemical surface etching(thermal oxidation), caused the nitride film to crack soon after thestart of operation. Cracking of nitride film necessitates replacementand cleaning of the inner tuber in the early stage and also gives offmany particles during operation.

[0154] Sample No. 12, which underwent thermal oxidation at a lowertemperature, has a less number of pits and a less total length ofelongated tiny depressions. Therefore, it caused the nitride film tocrack soon after the start of operation. Cracking of nitride filmnecessitates replacement and cleaning of the inner tuber in the earlystage and also gives off many particles during operation.

Example 3

[0155] Experiment No. 1

[0156] Samples of glass-like carbon tubes were prepared as follows froma commercial phenolic resin “PL4804” from Gunei Kagaku Kogyô Co., Ltd.,which had previously been dehydrated. The phenolic resin was made into atube by using a centrifugal molding machine equipped with the mold 10.Charged with a prescribed amount of the phenolic resin, the mold 10 wasrotated at 600 rpm for 24 hours, with its surface temperature kept at100° C. The mold was cooled to room temperature for demolding. Thusthere was obtained a phenolic resin tube 21 measuring 2.5 mm thick, 330mm in outside diameter, and 1200 mm long.

[0157] Then, the phenolic resin tube 21, with a core 23 inserted thereinas shown in FIG. 7, was carbonized at 1200° C. in the following manner.(This core is covered with carbon fiber felt.) The phenolic resin tube21, together with the core 23, was heated in an electric furnace filledwith nitrogen gas, with the temperature therein raised to 1200° C. at arate of 2° C./h. The phenolic resin tube 21 was kept at 1200° C. for 1hour for carbonization. The core 23 was removed. Thus there was obtaineda glass-like carbon tube 22 measuring 1.9 mm thick, 249 mm in outsidediameter (assuming a round tube), and 900 mm long. Incidentally, thecore 23 is a graphite tube 23 a, measuring 15 mm thick, 240 mm inoutside diameter, and about 1200 mm long. It has one layer of carbonfiber felt (3 mm thick) 23 b surrounding its outer surface, as shown inFIG. 7.

[0158]FIG. 8 shows one example of the roundness correcting jig used inthe present invention. FIG. 8(a) is a plan view, and FIG. 8(b) is asectional view taken along the line A-A. The roundness correcting jig ismade of graphite. As shown in FIG. 8(a), it is a square plate measuring300×300 mm and 30 mm thick. It has a round hole 24 a, 250 mm indiameter, at its center. Into this hole 24 a is inserted the glass-likecarbon tube 22 mentioned above.

[0159] As FIG. 9 shows, the roundness correcting jig 24 was arranged atthe outside of one end of the glass-like carbon tube 22, and theresulting assembly was heated in an electric furnace forhigh-temperature treatment in nitrogen gas. The temperature was raisedto 1200° C. at a rate of 20° C./h and then to 1500° C. at a rate of 2°C./h. This temperature was kept for 1 hour. After this high-temperaturetreatment, the roundness correcting jig 24 was removed.

[0160] The resulting glass-like carbon tube underwent sand blasting with#400 alumina powder for mechanical surface roughening on both the innerand outer surfaces thereof. Subsequently, it underwent purifyingtreatment by heating at 2200° C. in a chlorine-containing gasatmosphere.

[0161] After purifying treatment, the glass-like carbon tube wasexamined for roundness. It was found that there is a difference of 0.3mm between the maximum outside diameter and the minimum outside diametermeasured at the end at which the roundness correcting jig was arranged.The roundness correcting jig proved useful in production of alarge-scale glass-like carbon tube with a high degree of roundness. Theround glass-like carbon tube does not need grinding for dimensionaladjustment; it can be used as an inner tube of CVD apparatus.

[0162] Experiment No. 2

[0163] A phenolic resin tube, measuring 2.5 mm thick, 330 mm in outsidediameter, and 1200 mm long, was prepared in the same way as inExperiment No. 1. It underwent heat treatment, with a core 23 arrangedtherein, in an electric furnace filled with nitrogen gas. Thetemperature was raised to 1500° C. at a rate of 2° C./h; Thistemperature was kept for 1 hour. The resulting glass-like carbon tubeunderwent mechanical surface roughening and purifying treatment in thesame way as in Experiment No. 1.

[0164] The thus obtained glass-like carbon tube was found to have adifference of 1.2 mm between the maximum outside diameter and theminimum outside diameter. It is inferior in roundness to the sample inExperiment No. 1 and hence it needs to be machined before it is used asan inner tube.

[0165] [Effect of the Invention]

[0166] As mentioned above, the present invention provides an inner tubeof glass-like carbon for CVD apparatus and a process for productionthereof. The inner tube has its surface roughened without increase inmetal impurities which cause particles. The roughened surface improvesadhesion to the CVD deposit film. The surface roughening does not giveoff dust. Because of these features, the inner tube obviates thenecessity of frequent cleaning to remove CVD deposits therefrom. Inaddition, the glass-like carbon tube has a high degree of roundness, sothat it is suitable for CVD apparatus.

What is clamed is:
 1. A component of glass-like carbon for CVD apparatuswhich is characterized by having a value of surface roughness (R_(a))ranging from 0.1 to 10 μm (measured according to JIS B0601) andcontaining in its surface iron, copper, chromium, sodium, potassium,calcium, magnesium, and aluminum each in an amount less than 5×10¹⁰atoms/cm².
 2. The component of glass-like carbon for CVD apparatus asdefined in claim 1, which has a surface finished such that there existat least five pits, 1-10 μm in diameter, in the visual field, 50×50 μm,observed under a scanning electron microscope with a magnification of×1000.
 3. The component of glass-like carbon for CVD apparatus asdefined in claim 1, which has a surface finished such that there existelongated tiny depressions, 0.5-5 μm wide, whose total length is atleast 50 μm, in the visual field, 50×50 μm, observed under a scanningelectron microscope with a magnification of ×1000.
 4. The component ofglass-like carbon for CVD apparatus as defined in claim 1, which is anyof inner tube, wafer boat, susceptor, and nozzle used for CVD apparatus.5. A process for producing the component of glass-like carbon for CVDapparatus as defined in claim 1, comprising steps of: molding an objectfrom a raw material resin, carbonizing the resulting molded object togive a molded object of glass-like carbon, roughening the surface of themolded object, and purifying the molded object of glass-like carbon. 6.The production process as defined in claim 5, wherein roughening thesurface of the molded object is performed after any step between themolding step and the carbonizing step.
 7. The production process asdefined in claim 5, wherein purifying the molded object of glass-likecarbon is performed at any step after the surface roughening treatment.8. The production process as defined in claim 5, wherein the surfaceroughening treatment is mechanical one.
 9. The process as defined inclaim 5, wherein the surface roughening treatment is a combination ofmechanical one and chemical etching one, which are performedsequentially (in the order mentioned) or simultaneously.
 10. Theproduction process as defined in claim 8, wherein the mechanical surfaceroughening treatment is sandblasting or grinding.
 11. The productionprocess as defined in claim 9, wherein the chemical etching treatment isthermal oxidation or electrolytic oxidation.
 12. The production processas defined in claim 8, wherein the component of glass-like carbon forCVD apparatus is an inner tube and the mechanical surface rougheningtreatment is performed on both the inner and outer surfaces of the innertube all at once.
 13. The production process as defined in claim 5,wherein the purifying step is heat treatment at a high temperature in ahalogen-containing gaseous atmosphere.
 14. A process for producing thecomponent of glass-like carbon for CVD apparatus as defined in claim 1,with the component of glass-like carbon being an inner tube, saidprocess comprising steps of molding a raw material resin into a tube,heating the resulting resin tube at 800-1300° C., thereby converting itinto a glass-like carbon tube, and heating the glass-like carbon tube ata temperature higher than 1500° C., together with a roundness correctingjig attached to the outside of the glass-like carbon tube.